1. Field of the Invention
This invention relates generally to impedance matching transformers and, more particularly, to a shim-tuned coaxial cable impedance transformer.
2. Description of the Related Art
A generator, such as a transmitter, for example, is typically designed to operate into a specific impedance of a network. However, a load (e.g., an antenna) that is coupled to the generator usually does not provide the specific impedance in which the generator is designed to operate.
When the impedance of the load and the impedance as seen by the generator are equal, maximum power is transferred from the generator to the load over a transmission line coupling the generator to the load. If a mismatch between the impedances of the load and generator occurs, however, the power that is not transferred to the load may be returned towards the generator through the transmission line. These rearward-traveling waves may combine with their respective forward-traveling waves along the transmission line, and because of the phase differences along various positions within the line, may cause standing waves in the transmission line by the alternate cancellation and reinforcement of the voltage and current distributed along the transmission line. The larger the standing waves that occur along the transmission line, the greater the mismatch of the impedance of the load that is coupled to the generator.
In an attempt to compensate for this impedance mismatch between the generator and the load, series-tuned transformers, such as slug-tuned transformers, for example, have been used. These particular transformers, however, have been historically difficult to accurately construct and calibrate, thus resulting in a very limited improvement, if any, in impedance matching a generator to a load. Slug-tuned transformers are typically problematic because relatively large frequency shifts make it very difficult to match high standing wave ratio (SWR) values of the transmission line. Additionally, the slugs within the slug-tuned transformers cannot be changed or adjusted within the transformer without disassembly of the transformer. Accordingly, the slug-tuned transformer is difficult to calibrate as a result of the need to disassemble the transformer to replace and/or adjust the slugs.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.
One aspect of the present invention is seen in a transformer for matching the impedance of a generator and a load coupled to the generator via a transmission line. The transformer includes an outer conductor having an inner surface and an inner conductor positioned within the outer conductor. The transformer further includes at least one shim disposed on the inner surface of the outer conductor and encircling the inner conductor. The at least one shim is slideable along the inner surface of the outer conductor for matching the impedance of the generator and the impedance of the load.
Another aspect of the present invention is seen in a system. The system comprises a generator for generating a signal and a load for receiving the signal generated by the generator. The system further includes a transformer coupled between the generator and the load. The transformer includes an outer conductor having an inner surface and an inner conductor positioned within the outer conductor. The transformer further includes at least one shim disposed on the inner surface of the outer conductor and encircling the inner conductor. The at least one shim is slideable along the inner surface of the outer conductor for matching the impedance of the generator and the impedance of the load.
Another aspect of the present invention is seen in a method for matching the impedance of a generator to a load coupled to the generator via a transmission line. The method comprises providing an outer conductor having an inner surface and providing an inner conductor positioned within the outer conductor. The method further comprises providing at least one shim disposed on the inner surface of the outer conductor and encircling the inner conductor, the at least one shim being slideable along the inner surface of the outer conductor for matching the impedance of the generator and the impedance of the load.